Carbon Dioxide and Water Vapor Decrease the Alveolar Oxygen. Even at high altitudes, carbon dioxide (CO2) is continually excreted from the pulmonary blood into the alveoli. In addition, water vaporizes into the inspired air from the respiratory surfaces. These two gases dilute the O2 in the alveoli, thus reducing the O2 concentration. Water vapor pressure in the alveoli remains at 47 mm Hg as long as the body temperature is normal, regardless of altitude.
In the case of CO2, during exposure to very high alti tudes, the alveolar partial pressure of CO2 (PCO2) falls from the sea-level value of 40 mm Hg to lower values. In the acclimatized person, who increases ventilation about fivefold, the PCO2 falls to about 7 mm Hg because of increased respiration.
Now let us see how the pressures of these two gases affect the alveolar O2. For instance, assume that the barometric pressure falls from the normal sea-level value of 760 mm Hg to 253 mm Hg, which is the usual measured value at the top of 29,028-foot Mount Everest. Forty seven mm Hg of this must be water vapor, leaving only 206 mm Hg for all the other gases. In the acclimatized person, 7 mm Hg of the 206 mm Hg must be CO2, leaving only 199 mm Hg. If there were no use of O2 by the body, one fifth of this 199 mm Hg would be O2 and four fifths would be nitrogen; that is, the PO2 in the alveoli would be 40 mm Hg. However, some of this remaining alveolar O2 is continually being absorbed into the blood, leaving about 35 mm Hg O2 pressure in the alveoli. At the summit of Mount Everest, only the best of acclimatized people can barely survive when breathing air. However, the effect is very different when the person is breathing pure O2, as we see in the following discussions.
Alveolar Po2 at Different Altitudes. The fifth column of Table 1 shows the approximate PO2 values in the alveoli at different altitudes when one is breathing air for both the unacclimatized and the acclimatized person. At sea level, the alveolar PO2 is 104 mm Hg. At 20,000 feet altitude, it falls to about 40 mm Hg in the unacclimatized person but only to 53 mm Hg in the acclimatized person. The reason for the difference between these two is that alveolar ventilation increases much more in the acclimatized person than in the unacclimatized person, as we discuss later.
Table1. Effects of Acute Exposure to Low Atmospheric Pressures on Alveolar Gas Concentrations and Arterial Oxygen Saturation
Saturation of Hemoglobin with Oxygen at Different Altitudes. Figure 1 shows arterial blood O2 saturation at different altitudes while a person is breathing air and while breathing O2. Up to an altitude of about 10,000 feet, even when air is breathed, the arterial O2 saturation remains at least as high as 90 percent. Above 10,000 feet, the arterial O2 saturation falls rapidly, as shown by the blue curve of the figure, until it is slightly less than 70 percent at 20,000 feet and much less at still higher altitudes.
Fig1. Effect of high altitude on arterial oxygen saturation when breathing air and when breathing pure oxygen.
